The selective catalytic reduction (SCR) of nitrogen oxides produced during combustion processes using reductants such as NH3 has been a commercially successful technology for over 30 years. It was originally introduced for the control of NOx emissions in exhaust gases from stationary power plants and other industrial facilities. Recently, interest in the technology has expanded as the result of its utility for treatment of emissions from mobile power sources, such as marine vessels, cars, trucks, and machinery. This increased interest is driven largely by regulations that govern emissions from mobile sources. For example, the US EPA regulations that will be effective in 2010 for mobile diesel engines set such low emissions levels for NOx that efficient exhaust after-treatment is essential, and SCR is a leading technological choice.
In stationary applications, the requirements on the catalyst are not very high. For example, stationary engines typically run at close to steady-state, constant temperature conditions, and with relatively low gas space velocity. Further, the volumetric requirements for the catalyst are not too demanding. In on-road mobile applications, however, the catalysts requirements are much more severe. In this case, engines are not run at steady-state or at constant temperature, but instead cycle over wide variations in load (and hence temperature). In one possible system configuration, the SCR catalyst is located downstream of a diesel particulate filter (DPF), and regeneration of the soot-loaded DPF can cause a high temperature pulse of hot gas to pass through the downstream SCR catalyst. Furthermore, the mobile applications typically involve much higher gas space velocities and the volumetric requirements on the catalyst are severe. For example, in early application of SCR applications to heavy-duty diesel engines, the catalyst volume was several times greater than the engine displacement! For these reasons, it is imperative that improved catalysts be developed that have higher thermal stability, and improved volumetric activity, so that cost-effective technological solutions can be found to meet increasingly strict regulations.
The technology that has been utilized for many years in stationary applications involves catalysts based on metal oxides, and especially those based on TiO2 as the catalyst support, and the active catalytic functionality is based on vanadia, V2O5. Thus, mixtures of TiO2 (80-95%), WO3 (3-10%) and optionally with the balance comprising SiO2 (such as DT-52™ and DT-58™) have been in use as the catalyst support, and the active vanadia component is typically present at 0.1 to 3 wt %. In these catalysts the titania is initially present with relatively high surface area in the anatase form. The use and limitations of vanadia-based catalysts for mobile urea-SCR systems are reviewed in “Studies in Surface Science and Catalysis”, Granger, P. and Parvulescu, V. I., ed., Vol. 171, Chapter 9. There are two considerations that rely on greater stability of the vanadia-based catalyst. First, the catalysts may be used in mobile applications in a configuration where a diesel particulate filter (DPF) is positioned upstream of the vanadia-SCR catalyst. In this configuration, the vanadia catalyst may be exposed to extremes in temperature associated with the exothermic regeneration of the DPF. A second consideration is that it is desirable for a vanadia-based catalyst to maintain its catalytic activity at high temperatures (e.g., >550° C.) so as to better compete with base-metal exchanged zeolite catalysts, which show a high degree of stability and activity at high temperatures. DT-58™ contains 10 wt % SiO2 9 wt % WO3 and 81% TiO2, and has a surface area of about 90 m2/gm. It is well known, however, that catalysts based on titania and vanadia are not particularly thermally stable. There are several reasons for this lack of thermal stability. First, the titania by itself tends to sinter at elevated temperature, with an associated loss of surface area. Second, the titania also undergoes a transformation to the rutile crystalline form at high temperatures, and this form is generally thought to be a less active support than the anatase form. Third, unsupported vanadia has a melting point of about 675° C., and so, even when it is supported on titania, at elevated temperatures it tends to be somewhat mobile and can eventually aggregate to form low surface area (and less active) vanadia crystals.
For these reasons, it is imperative to improve the thermal stability of the final catalyst, and at the same time, maintain or increase the catalytic activity for selective catalytic reduction of nitrogen oxides (SCR-DeNOx) from lean-burn mobile engines. Achieving both goals simultaneously is a significant challenge, since often one can be improved at the detriment of the other. For example, the incorporation silica and/or rare-earths into the titania is reported to increase stability, but further gains in both stability and activity are needed.
Amorphous silica-stabilized ultrafine anatase titania have previously been used in catalytic applications. It is known that amorphous silica improves the anatase phase stability and surface area retention of ultrafine anatase titania, and hence amorphous silica is an additive in commercial products like DT-58™ and DT-S10™, and these materials can be used commercially in selective catalytic control catalysis of diesel emissions, particularly for DeNOx applications.
An early patent describes the use of “silicic acid” to stabilize anatase titania for DeNOx (U.S. Pat. No. 4,725,572). However, a careful reading of this patent shows that the silica source is in fact a colloidal, particulate silica. A more recent U.S. patent (U.S. Pat. No. 6,956,006 B1) also describes the use of colloidal silica to effect an anatase titania with enhanced thermal and hydrothermal stability. A recent published U.S. patent application (U.S. 2007/0129241 A1) discusses vanadia/titania-based DeNOx catalysts with improved stability. The silica source used therein is also a colloidal silica. However, these colloidal silica-based titania catalysts, as noted, lack stability and acceptable activity after extremes of high temperature. Titania catalysts which minimize these shortcomings would be of great use and advantage.
While the DT-58™ support material mentioned above is a state-of-the-art support material for diesel emission catalysts, an improved titanium support would, in general, be (1) more thermally stable, thus enabling its placement in closer proximity to the engine, and (2) more catalytically active, thus enabling use of a smaller canister (say 10 L vs. 12 L) for containing the catalyst, thus optimizing (reducing) the size of the emission control system.
It is to the production of such improved silica supported-titania substrates, and catalysts made therefrom, that the present invention is directed.